This invention relates to cathode collectors for liquid cathode electrochemical cells and particularly to a method of forming improved porous carbon cathode collectors as for lithium-thionyl chloride cells.
Liquid cathode cells such as thionyl chloride cells, which utilize molded cathode collectors, are particularly useful when employed at low rates of discharge, i.e. at low cathode current densities. However, such cells exhibit very poor performance at high rates of discharge. The cells typically cease effective function after a relatively short time, when a majority of the anode is still present and the cell is still wet with catholyte (cathode-electrolyte). Consequently, there is considerable waste of active materials resulting, as well as short cell life and the need for frequent replacement of the cells.
By analysis of the problem, the inventors herein suspected that one possible cause of this could be plugging of the cathode collector with solid reaction products. Subsequent experimentation, examination and analysis resulted in the conclusion that the cathode collector regions closest to the anode did exhibit plugging. This was considered to be interfering with cell functioning.
The inventors then conceived and developed a novel method of dry molding a cathode collector with particular pore characteristics resulting in a superior collector resistant to the plugging problem that normally unduly shortens the lifetime of such cells, thus making possible the construction of liquid cathode cells with molded cathode collectors having greatly improved performance at high cathode current densities.
Carbon cathode collectors are typically formed into their molded shapes by wet molding processes which involve forcing an appropriately shaped ram into the wet flowable mix of carbon and binder retained in the cell container, thereby forcing the wet mix to flow up into the space between the ram and the container wall. The mix is then dried. Johnson et al, U.S. Pat. No. 3,985,573, discloses such a method. Johnson et al also suggested (column 8, lines 32-40) the technique of substantial removal of water from a slurry, followed by sintering into a cake, breaking up the sintered cake into fine particles of powder, and molding such into the form of a slotted annular bobbin.